x86: Optimize {str|wcs}rchr-avx2

The new code unrolls the main loop slightly without adding too much
overhead and minimizes the comparisons for the search CHAR.

Geometric Mean of all benchmarks New / Old: 0.832
See email for all results.

Full xcheck passes on x86_64 with and without multiarch enabled.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

(cherry picked from commit df7e295d)